What is causing the acceleration of the expansion of our Universe? Even after twenty-five years since its discovery, this phenomenon remains one of the most puzzling mysteries in science. Understanding it requires a reevaluation of fundamental physical laws, including Albert Einstein’s theory of general relativity. Researchers from the universities of Geneva (UNIGE) and Toulouse III — Paul Sabatier analyzed data from the Dark Energy Survey and found a small inconsistency that shifts across different eras in cosmic history. These findings, published in Nature Communications, raise questions about the applicability of Einstein’s theories when explaining events beyond our solar system on a universal scale.
According to Einstein’s theory, the Universe can be thought of as a flexible sheet deformed by matter. These deformations, influenced by the gravitational pull of celestial bodies, are termed ”gravitational wells”. When light travels through this curved framework, its path is altered by these gravitational wells, similar to how a glass lens would bend light. In this scenario, it’s gravity – not glass – that causes the light to bend. This effect is referred to as ”gravitational lensing”.
By observing this effect, scientists gain valuable insights into the Universe’s components, its history, and the nature of its expansion. The first evidence for gravitational lensing was recorded during a solar eclipse in 1919, which validated Einstein’s theory; it predicted a deflection of light that was double what Isaac Newton had anticipated. This discrepancy is due to Einstein’s revolutionary concept of time deformation, alongside space deformation, allowing for a precise curvature of light.
Theory vs. Data
Are Einstein’s equations still applicable at the Universe’s boundaries? This question drives many researchers who are working to assess cosmic matter density and comprehend the accelerated expansion. Using data from the extensive mapping project of the Dark Energy Survey, a team from UNIGE and Toulouse III — Paul Sabatier is shedding new light on these questions.
”Until now, the Dark Energy Survey’s data were primarily utilized to examine the distribution of matter in the Universe. In our approach, we utilized this information to assess time and space distortions directly, allowing for a direct comparison with Einstein’s predictions,” states Camille Bonvin, an associate professor in the Department of Theoretical Physics at UNIGE, who spearheaded the research.
A Small Discrepancy
The Dark Energy Survey enables scientists to look deep into space and thus back in time. The French-Swiss team investigated 100 million galaxies at four different points in the Universe’s timeline: 3.5, 5, 6, and 7 billion years ago. Their analysis revealed how the gravitational wells have transformed over this extensive span of cosmic history.
”We observed that in the more distant past – specifically 6 and 7 billion years ago – the gravitational wells’ depth was consistent with Einstein’s predictions. However, as we get closer to the present, around 3.5 and 5 billion years ago, the wells appear to be somewhat shallower than what Einstein predicted,” explains Isaac Tutusaus, an assistant astronomer at the Institute of Research in Astrophysics and Planetology (IRAP/OMP) at Université Toulouse III – Paul Sabatier and the lead author of the study.
This period, closer to the present, corresponds with the start of the Universe’s accelerated expansion. Thus, there may be a link between two phenomena – the Universe’s acceleration and the shallower growth of gravitational wells – suggesting that gravity may behave under different physical rules on vast cosmic scales compared to what Einstein proposed.
Is Einstein’s Theory in Question?
”Our findings indicate that there is a 3 sigma inconsistency with Einstein’s predictions, which sparks interest in the scientific community and calls for further exploration. However, this level of inconsistency is not sufficient to disprove Einstein’s theory at this time. To reach that conclusion, we would need to achieve a 5 sigma threshold. Therefore, gathering more accurate data is crucial to either support or challenge these preliminary results and determine if Einstein’s theory holds at extensive distances in our Universe,” notes Nastassia Grimm, a postdoctoral researcher in the Department of Theoretical Physics at UNIGE and co-author of the study.
The team is preparing to analyze fresh data from the Euclid space telescope, which was launched a year ago. With its vantage point in space, Euclid’s observations of gravitational lensing will be notably more accurate. It is anticipated that the mission will survey approximately 1.5 billion galaxies over six years. This enhanced observation capability will facilitate better measurements of space-time distortions, allowing researchers to delve deeper into the past and ultimately test the viability of Einstein’s equations.
